In a optical telephone it is required to convert amplitude modulated light to sound or mechanical energy. Most often this is done by converting the modulated light into electrical energy and then feeding this to an electro-mechanical transducer or earphone. The "Photophone" patented by Bell and Tainter, does this using the temperature/volume characteristics of a gas. The modulated light (varying optical energy) is applied to an enclosed volume of special gas. The volume of this gas varies in synchronism with instantaneous energy of the modulated light. In this variation, the sounds are produced. This is likely to be an inefficient mechanism.